Belarusian world number seven Sabalenka, who finished the group stage with a 2-1 record, advances if Sakkari wins one set. If the Greek drops a set, or even loses in three sets, there are still mathematical possibilities for Sabalenka to progress.
Sabalenka made a blistering start, breaking Pegula to open the match and again for a 5-2 lead.
World number three Pegula then fought off four set points to break straight back but Sabalenka broke to love in the next game to wrap up the set.
Pegula fell behind 2-0 in the second but fought back before Sabalenka, at 3-3, delivered a love hold and then came back from 40-15 down for a crushing service break before the top-ranked American returned the favour to extend the set.
But Sabalenka held to love for the third time in the set and fourth in the match for a 6-5 lead and then closed it out on her second match point when she ripped a backhand crosscourt winner.
"She's an amazing player and she made me really work tonight," Sabalenka said in her on-court interview.
"Just happy that I was really focused from the beginning to the end and I was able to finish this match in two sets."
World number two Jabeur of Tunisia and world number five Sakkari are scheduled to begin their match at 8pm ET (1am CET).
